1. Field of the Invention
The present invention relates to a radio transmitter suitable for microwave and satellite communication systems and, more particularly, to a radio transmitter of the type effecting multicarrier TDMA (Time Divison Multiple Access) communication by frequency hopping.
2. Description of the Related Art
One of conventional radio transmitters of the type described has a direct digital frequency synthesizer (DDS)and a phase locked oscillator. The DDS generates a frequency hopping signal lying in the VHF band by hopping the frequency in a small frequency step and at high speed. The phase locked oscillator removes spurious signal components from the frequency hopping signal and produces a local oscillation signal whose frequency is an integral multiple (e.g. microwave band) of the frequency of the frequency hopping signal. A mixer combines the frequency of the local oscillation signal and the frequency of a modulated signal of intermediate frequency (IF) band to produce a transmit signal to be sent from the transmitter. The modulated signal is produced by modulating the phase of an IF carrier signal by, for example, a digital modulating signal.
The problem with such a conventional radio transmitter is that the transmitter is scaled up since it needs a band-pass filter for removing a needless wave signal ascribable to the mixer from the transmit signal. Moreover, when the hopping frequency range of the frequency hopping signal is broad, the frequency of the transmit signal and that of the needless wave signal are apt to approach each other. Then, the radio transmitter is likely to fail even to remove the needless wave signal.
It has been customary to provide the phase locked oscillator with a local oscillation signal generated by a voltage controlled oscillator having an oscillating element implemented by a transistor. However, when the loop band of the phase locked loop is reduced to remove the spurious signal components of the local oscillation signal, it is likely that the requirement on the phase noise level of the output local oscillation signal cannot be satisfied.